(a) Field of the Invention
The present invention relates to a Multiple Input Multiple Output (MIMO)—Orthogonal Frequency Division Multiplexing (OFDM) system. More specifically, the present invention relates to a MIMO-OFDM system using eigenbeam forming in a downlink.
(b) Description of the Related Art
A beam forming method has been used to obtain the antenna array gain to improve performance. Also, the beam forming method can be used to use a space domain in a downlink channel of a MIMO system. Generally, a base station is required to have instantaneous channel information in a downlink to apply a closed-loop downlink beam forming method. In a Frequency Division Duplexing (FDD) mode, a mobile station is required to feedback the instantaneous information to the base station, since frequency bands are different between uplink channel and downlink channel. Here, when the amount of the feedback information is large, the feedback information hinders the closed loop beam forming. Thus, a method capable of reducing the feedback information is required to be investigated.
The blind beam forming method adaptively forms a downlink beam by measuring an uplink channel, under the assumption that spatial statistical properties of the channels are similar, since constructions for transferring conditions are similar in both uplink and downlink. The method does not require feedback information since the method uses reciprocity of the channels; however the method does not satisfy diversity gain since the beam forming vector does not include the instantaneous channel variation. To obtain space diversity gain, it is necessary to feed back the instantaneous channel information in the downlink. Here, the amount of feedback information increases, and the feedback rate for tracking channel variation increases when the number of transmit antennas increases. Thus, it is difficult to apply the beam forming method when the number of transmit antennas is large or the speed of the mobile station is high. To solve the above problems, several methods are proposed, as follows.
An eigenbeam forming method proposed by the 3rd Generation Partnership Project (3GPP) uses spatial correlation and selection diversity. The spatial correlation can allows long-term feedback with much feedback information, and the selection diversity can requires a very small amount of feedback information when short-term feedback is required in accordance with the instantaneous channel variation. That is, according to the eigenbeam forming method, the mobile terminal finds a dominant eigenmode by using a spatial covariance matrix of which a short term update is not necessary and feeds back the dominant eigenmode; and the mobile terminal feeds back the strongest eigenmode in the uplink by using the instantaneous channel variation among dominant eigenmodes. The base station selects the strongest eigenmode and transmits the signals. Thus, the eigenbeam forming method can obtain the selection diversity gain in addition to the signal-to-noise ratio gain.
From the situation that the antenna array of the base station is generally located on the top of some buildings, the down link channel exhibits to have a high spatial correlation or few dominant eigenmodes. Since there are no local scatters around the antenna array of the base station, the signal can be spatially selectively transmitted with only few directions. It is regarded that the eigenmode generates an independent path between the base station and the mobile station. The eigenbeam forming method can be effectively used in this condition.
However, when the eigenbeam forming method is applied to the OFDM system, each subcarrier of OFDM is selectively faded at different frequencies in the OFDM system. Thus, each subcarrier has a different beam forming vector and all subcarriers are required to feed back their beam forming vectors. In this case, the amount of the feedback information becomes very much larger than that of the single subcarrier, and the feedback information provides a severe burden to the system.